1. Field of the Invention
This invention relates to a radio communication system such as a digital cellular phone communication system and digital radio LAN, especially, to a system using TDD scheme as a radio access scheme.
2. Description of the Related Art
For example, in a digital mobile communication (cellular phone) system, FDD (Frequency division duplex) scheme and TDD (Time division duplex) scheme are used as a radio access scheme between the base station and mobile station. FDD scheme allows the full duplex communication by allocating a different radio carrier frequency to an uplink directed from the mobile station to a base station and to a downlink directed from the base station to the mobile station. Conversely, TDD scheme allows the full duplex communication by allocating a different time slot on the same radio carrier frequency to the uplink and the downlink.
FDD scheme must always maintain a pair of radio carrier frequencies. However, it is not required by the TDD scheme, thereby eliminating a duplexer with enhanced frequency utilization efficiency. Moreover, the TDD scheme has approximately the same radio propagation conditions, with an advantage of simplifying the configuration for diversity communication. For such reasons, the TDD scheme has been extensively applied to PHS (Personal Handy Phone System) and DECT (Digital European Cordless Telecommunications) in the digital mobile communication system.
Meanwhile, recently, a satellite-base mobile communication system has been proposed in which the radio communication between mobile stations or between a mobile station and base station is established via a communication satellite. This system is capable of covering a vast service area in comparison with the ground base mobile communication system. However, the satellite-base mobile communication system generally uses the FDD scheme as a radio access scheme and does not use the TDD scheme. The reason of this is as follows:
Thus, the TDD scheme alternately performs the sending and receiving operation so that it is necessary to accurately control the switching timing. In contrast to this, the satellite-base mobile communication system mainly uses a satellite moving along a low-altitude orbit. When a radio communication is established between mobile stations via an orbiting satellite, two-to three-folds difference will appear in a propagation delay time due to the position of the orbiting satellite. FIG. 14 shows the change in the propagation delay with respect to the altitude of the communication satellite. For example, suppose that there is a mobile station communicating with an orbiting satellite at the altitude of 1000 km. In this instance, the propagation delay time will be approximately 3.2 msec when the satellite is positioned just above the mobile station, namely, the distance between the mobile station and the satellite is the shortest, and on the other hand, the time will be 9 msec when the satellite is positioned on a horizontal point as viewed from the mobile station, namely, the distance between the mobile station and the satellite is the most distant. Thus, the propagation delay time between the mobile station and satellite will be up to three times longer depending on the position of the satellite. In addition to that, the time required for the satellite to move from the horizontal position viewed from the mobile station to the position just above the mobile station is only a few minutes. Moreover, the satellite-base mobile communication system is closely described, for example, in the document "Satellite Communications-Mobile and Fixed Service" written by M. J. Miller, D. Vacetic, L. Berry.
Thus, when one tries to apply the TDD scheme to the satellite-base mobile communication system whose propagation delay time changes extensively during communication, it makes necessary to accurately control the switching timing between the sending and receiving on the mobile station and satellite, with said propagation delay time taken into consideration, thereby making it very difficult to control the timing.
Moreover, it is conceivable to provide a large guard time between the sending time slot and the receiving time slot to simplify or dispense with the control of switching timing with said propagation delay time took into consideration. However, it does not provide an effective means since enlarged guard time results in the reduction in the propagation efficiency.